1. Field of the Invention
The invention relates to a cold cathode fluorescent lamp suitable for a liquid crystal display, and further to a back-light emitting device having the cold cathode fluorescent lamp, and still further to a note-type personal computer having the back-light emitting device.
2. Description of the Related Art
In these days, a liquid crystal display mounted on a note-type personal computer is required to have 800.times.600 pixels or 1024.times.768 pixels both having a high resolution. Along with an increase in a resolution, a liquid crystal display becomes larger in size. For instance, a size of a liquid crystal display screen has changed from 12.1 to 13.3, and then, from 13.3 to 14.1.
However, a note-type personal computer has a restriction that a size thereof does not exceed A4 size or A4 file size. In addition, a note-type personal computer is required to have a smaller weight. As a result, there are many problems to be solved with respect to a liquid crystal display to be mounted on a note-type personal computer.
The first problem is that a liquid crystal display has to have a smaller thickness and a smaller weight.
The second problem is that a distance between a liquid crystal screen and an outer edge of a liquid crystal display is shortened in order to make it possible to incorporate a larger screen into a limited floor area of a note-type personal computer. In other words, a smaller-framed screen is required.
The third problem is that an arrangement of parts except a liquid crystal display, such as an inverter, is altered to thereby prevent an increase in a floor area of a note-type personal computer.
FIGS. 1 and 2 partially illustrate conventional note-type personal computers.
The conventional note-type personal computer illustrated in FIG. 1 is comprised of a first body 16a including a structure acting as a computer and an input means such as a keyboard, a second body 16b including a display screen 17 which has an outer periphery 15 and on which images are displayed, a hinge structure 11 formed between the first and second bodies 16a and 16b for connecting the second body 16b to the first body 16a so that the second body 16b is rotatable relative to the first body 16a, an inverter 12 housed in the hinge structure 11 almost at the center, and a cold cathode fluorescent lamp 5 housed in the second body 16b at a bottom.
The cold cathode fluorescent lamp 5 includes first and second terminal electrodes 1 and 4 at opposite ends. The first terminal electrode 1 is electrically connected to a low voltage cable 10 through both a lead-in wire 6a and a thin wire 7, and the low voltage cable 10 is connected to a low voltage terminal 14 of the inverter 12. The second terminal electrode 4 is electrically connected to a high voltage cable 9 through a lead-in wire 6b, and the high voltage cable 9 is connected to a high voltage terminal 13 of the inverter 12.
The conventional note-type personal computer illustrated in FIG. 2 has the same structure as that of the note-type personal computer illustrated in FIG. 1. The note-type personal computer illustrated in FIG. 1 is different from the note-type personal computer illustrated in FIG. 2 with respect to a location of a wire port 8 through which low and high voltage cables 9 and 10 extend. Specifically, the second body 16b of the note-type personal computer illustrated in FIG. 1 is formed at a bottom corner with the wire port 8, whereas the second body 16b of the note-type personal computer illustrated in FIG. 2 is formed at a center of a bottom edge with the wire port 8.
The reasons why it is difficult to render a frame around the display screen smaller are in a conventional note-type personal computer as follows. If a frame around the display screen is made smaller, the cold cathode fluorescent lamp 5 is located just in the close vicinity of, or at the rear of the display screen 17. Hence, when the cold cathode fluorescent lamp 5 is turned on, fluorescent lights pass directly through the display screen 17. In addition, there has to exist a space just below the display screen 17 for housing therein wires connecting the first and second terminal electrodes 1 and 4 to the inverter 12. Hence, the cold cathode fluorescent lamp cannot avoid to be located closer to the display screen 17 by a distance corresponding to the above-mentioned space, which makes it more difficult to form the frame smaller.
As a solution to the above-mentioned problems, there is employed the thin wire having a diameter of about 0.3 mm for connecting the lead-in wire 6a and the low voltage cable 10, to thereby narrow the above-mentioned space for locating the cold cathode fluorescent lamp 5 remoter from the display screen 17.
If the display screen 17 is made larger in size, a back-light emitting device has to be made larger accordingly, and as a result, a cold cathode fluorescent lamp as a back-light source has to be made longer accordingly.
A cold cathode fluorescent lamp is presently widely used as a back-light source for a liquid crystal display, because a cold cathode fluorescent lamp has many advantages that it generates a small amount heat, it has a relatively long lifetime, and an electrode structure is simple, and hence is able to be formed smaller, contributing to formation of a liquid crystal display in a smaller size.
However, if a cold cathode fluorescent lamp were designed to have a smaller diameter and a longer length, a break-down voltage and a discharge voltage would be both increased. Specifically, if a display screen has a width across corners of 14 inches, a cold cathode fluorescent lamp would have a length exceeding 280 mm, and a break-down voltage and a discharge voltage of a cold cathode fluorescent lamp having a diameter of 2.0 mm would reach about 1200 Vrms and 650 Vrms, respectively.
A hot cathode fluorescent lamp has a lower discharge voltage than that of a cold cathode fluorescent lamp, but has shortcomings that a filament electrode emitting thermoelectrons which cause light-emission generates heat, a hot cathode fluorescent lamp cannot be formed smaller in diameter because electrodes cannot be formed smaller in size, and a hot cathode fluorescent lamp has a short lifetime. Accordingly, a hot cathode fluorescent lamp is scarcely used as a backlight source of a liquid crystal display used for a note-type personal computer.
As mentioned earlier, the note-type personal computer illustrated in FIG. 1 employs the thin wire 7 for connecting the lead-wire 6a to the low voltage cable 10 in order to make a frame around the display screen 17 smaller. However, since the high and low voltage cables 9 and 10 are designed to extend through the wire port 8 formed a t a corner of the second body 16b, there is paused a problem that those high and low voltage cables 9 and 10 cause the second body 16b larger in size.
The reason is as follows. The high voltage cable 9 has to have a high resistance to high voltages, and hence, cannot avoid to have a relatively large diameter. For this reason, if the wire port 8 through which the high voltage cable 9 is introduced is formed at a corner of the second body 16b, it would be necessary to make a space A between the second body 16b and the outer periphery 15 of the display screen 17 for housing the cables 9 and 10 therein. As a result, the second body 16b cannot avoid to become larger in size to a degree corresponding to the space A.
In the note-type personal computer illustrated in FIG. 2, the wire port 8 through which the high and low voltage cables 9 and 10 are introduced is formed at a center of a bottom edge of the second body 16b. Hence, a space for housing the high and low voltage cables 9 and 10 therein, such as the space A illustrated in FIG. 1, is cancelled by the hinge structure 11, and thus, the above-mentioned problem about the space A is solved in the note-type personal computer illustrated in FIG. 2.
However, the note-type personal computer illustrated in FIG. 2 is accompanied with a problem that it is impossible to form a frame around the display screen 17 smaller due to the formation of the wire port 8 at the center of the bottom edge of the second body 16b.
The reason is as follows. The high voltage cable 9 is required to have a relatively large diameter in order to withstand high voltages. Hence, the note-type personal computer has to form a space B for housing the high voltage cable 9 therein. The space B is longer than the space A illustrated in FIG. 1. Hence, the second body 16b cannot avoid to become larger in size to a degree corresponding to the space B.
As explained so far, it is quite difficult or almost impossible in the conventional note-type personal computer to concurrently accomplish formation of a smaller frame around the display screen 17 and prevention of the second body 16b from becoming larger in size.
In addition, if the cold cathode fluorescent lamps used in the conventional note-type personal computer illustrated in FIGS. 1 and 2 are formed long, it would be difficult to design an insulating structure around the electrodes 1 and 4, and make the inverter 12 in a smaller size.
The reason is as follows. If a cold cathode fluorescent lamp is formed long, a break-down voltage and a discharge voltage are both increased, resulting in that discharged electrons tend to be attracted to a metal located in the vicinity of the cold cathode fluorescent lamp. Thus, it would be quite difficult to completely insulate the electrodes from surroundings.
In addition, the inverter 12 has to have a great step-up ratio in order to emit a greater output voltage. A step-up ratio of an electromagnetic transformer is in dependence on the number of turns of copper wires wound around a core.
Hence, if a step-up ratio is to be increased, the number ratio of copper wire turns becomes greater, resulting in that an electromagnetic transformer cannot avoid becoming larger in size.
Japanese Unexamined Utility Model Publications Nos. 6-84670 and 6-84671 have suggested a multi-electrode fluorescent lamp, which is illustrated in FIG. 3. The suggested multi-electrode fluorescent lamp is comprised of a glass tube 21 having a main portion 21a and a projected portion 21b, a first terminal electrode 1 fixed at an end of the main portion 21a by means of a first base 20a, a second terminal electrode 4 fixed at the other end of the main portion 21b by means of a second base 20b, an intermediate terminal 19 fixed at an end of the projected portion 21b by means of a third base 20c, and a first lead-in wire 6a connected to the first terminal electrode 1 through the first base 20a, a second lead-in wire 6b connected to the second terminal electrode 4 through the second base 20b, and a third lead-in wire 6c connected to the intermediate electrode 19 through the third base 20c.
The above-mentioned multi-electrode fluorescent lamp has a problem that the electrodes 1, 4, and 19 occupy a large space, which prevents a frame around the display screen 17 from becoming smaller.
The reason is as follows. As illustrated in FIG. 3, the intermediate electrode 19 is positioned in the projected portion 21b of the glass tube 21, and is fixed to the projected portion 21b by means of the third base 20c. The presence of the projected portion 21b and the third base 20c causes a frame around the display screen 17 to become larger in size.
In addition, above-mentioned multi-electrode fluorescent lamp further has a problem that it is quite difficult to design the lamp to have a smaller diameter, because the electrodes 1, 4, and 19 are in the form of a hot cathode fluorescent lamp.
The reason is as follows. An electrode used in a hot cathode fluorescent lamp is comprised of a filament electrode for emitting thermoelectrons. Hence, each of the bases 20a, 20b, and 20c have to have two pins as terminals to connect to the electrodes 1, 14, and 19, respectively. As a result, a large space is required to arrange the filament electrode and the associated base, and accordingly, it is difficult to make a diameter of the lamp smaller.
Japanese Unexamined Patent Publication No. 8-273604 has suggested a planar fluorescent lamp. FIG. 4 is a cross-sectional view of the suggested planar fluorescent lamp, and FIG. 5 is a cross-sectional view taken along the line V--V in FIG. 4.
The suggested planar fluorescent lamp is comprised of a hermetically sealed container 30, a first terminal electrode 1 having a length almost equal to a height of the container 30, and located at an end of the container 30, a second terminal electrode 4 having a length almost equal to a height of the container 30, and located at the other end of the container 30, a central electrode 19 having a length almost equal to a height of the container 30, and positioned at the center between the first and second terminal electrodes 1 and 4, lead-in wire pairs 6 each connected to the electrodes 1, 4, and 19 at opposite ends, an inverter 12, a high voltage cable 9 connecting the central electrode 19 to a high voltage terminal 13 of the inverter 12, and a low voltage cable 10 connecting the first and second terminal electrodes 1 and 4 to a low voltage terminal 14 of the inverter 12.
However, the above-mentioned planar fluorescent lamp is accompanied with a problem that it does not contribute to formation of a liquid crystal display in a smaller size and a smaller weight.
The reason is as follows. In general, a pressure in a fluorescent lamp is seven to eight times smaller than an atmospheric pressure. Specifically, a pressure in a fluorescent lamp is in the range of about 90 to about 100 Torr, whereas an atmospheric pressure (1 atm) is equal to 760 Torr. Hence, when a large surface light source is to be formed, it is necessary for both a front glass panel 22 and a rear glass panel 23 to have a certain thickness for having a sufficient strength in order to keep an inner gap of the container 30 constant, even if an external pressure acts on the container 30. As a result, a liquid crystal display including the container 30 having a thick outer wall and a heavy weight cannot be formed thinner and lighter.
Masaki Kinoshita has discussed characteristics required for a liquid crystal display in "Liquid Crystal with Back-Light required for Note-type Personal Computer", Monthly "Display", Vol.6, pp. 94-100, June 1997. According to this article, a back-light emitting device used for liquid crystal module is required to have a relatively long lifetime, a low power consumption rate, a smaller thickness, a smaller weight, and a smaller frame around a display screen. A minimum frame is about 4 mm.
Akio Obara has discussed requirements for a back-light emitting device, and compared a hot cathode fluorescent lamp to a cold cathode fluorescent lamp to be used for a back-light source, in "Status and Problems in Back-Light used for Liquid Crystal Display", Monthly "Display", Vol. 5, pp. 19-27, May 1996.